Archive for February 13th, 2012
Sleet Update – 530 pm
The leading edge of what will be a precipitation event that lasts several hours tonight has now pushed as far east as BHM. The precipitation is starting out in many places as sleet…and some of it is rather heavy. It is sleeting now here in Trussville, and reports of sleet have come in from all over western and central Alabama. The sleet briefly covers elevated surfaces in some spots…check out this picture from twitter of sleet covering a park bench at Univ of Alabama campus in Tuscaloosa.
It is fascinating that it is sleeting when the temperature was 49 at BHM at 5:00 pm, and 42 at TCL. This is a classic example of evaporational cooling. When the temp is 49 but the dewpoint is 17 degrees, the air has only 0.2% water vapor, but with a temp of 49, the air can hold 0.7% water vapor). So in every pound of air, the amount of water can be multiplied by 3.5 before we are saturated. So, as precipitation falls into a dry air mass like this, the evaporation of water cools the air. At the same time, it adds water to the air, raising the dew point. Where the two meet is the wet bulb temperature. This is the temperature we should settle out at once it rains/sleets for while. In BHM that temp is about 38. The rain is forming aloft in warmer air, then falling into the evaporatively cooled air near the surface and freezing, forming the ice pellets.
The sleet could last into the evening, but it will eventually turn over to all rain as warmer air moves in aloft, making the raindrops falling warmer and harder to freeze. We do not expect any significant travel problems with this event…surface temperatures probably won’t drop much below 40. But, in the heavier pockets of sleet, they could briefly cover a bridge and cause a slick spot for 5-10 minutes, so use extra care when driving this evening!
Sleet at 45 degrees? Expecting sleet when it’s nearly 50?
…FOR JAMES’ FULL FORECAST SCROLL DOWN A LITTLE…
Radar shows the beginnings of a large mass of precipitation moving into western Alabama. The air very dry over Alabama, so most of this precip (especially east of Tuscaloosa) is not reaching the ground.
The temperature in BHM is currently 49 degrees. But, the dewpoint is 15 degrees. This means the air has almost no moisture in it at all (it is 0.2% water vapor), but with a temp of 49, the air can hold 0.7% water vapor). So in every pound of air, the amount of water can be multiplied by 3 before we are saturated. So, as precipitation falls into a dry air mass like this, the evaporation of water cools the air and yet adds water to the air, raising the dew point. Where the two meet at equilibrium is the wet bulb temperature. This is the temperature we should settle out at once it rains/sleets for while. In BHM that temp is about 37. Here is a map:
And, the air 3,000 feet above the surface is even colder. So, as some of the warm rain formed up at 5,000 to 10,o00 feet falls initially, it will freeze on the way down in this wet-bulbing process and form sleet. This has already occurred over parts of NW Alabama.
Fortunately, surface temperatures are expected to stay above freezing (since the wet bulb is above freezing, so no widespread travel problems are expected in central AL. NWS Huntsville has issued a winter weather advisory, as they have wet bulbs near 34-35, and some of the valleys in NE Alabama, where this morning’s cold air got trapped, or the hilltops, probably have wet bulbs near 32. So, there may be a few slick spots up in extreme north and northeast Alabama this evening.
Once it starts raining harder overnight and warmer air moves in aloft, it should go to all rain in TCL/BHM/ANB. But, in NE Alabama, there still could be some icy spots by morning.
A Little Sleet; Mostly Rain Tonight
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THIS AFTERNOON: Despite temperatures in the 45-50 degree range, we have many reports of sleet across West Alabama as evaporative cooling keeps the small ice pellets intact on the way down to the surface. Needless to say, with surface temperatures so warm, there won’t be any driving issues tonight, and all of the precipitation will change to rain soon.
TO THE NORTH: The NWS in Huntsville has issued a winter weather advisory for their CWA (county warning area), but quite frankly even up there temperatures will be well above freezing and there should be no travel issues, despite a light wintry mix as the precipitation begins this evening.
Rain amounts overnight will be light, generally under 1/2 inch, and under 1/4 inch in many places. The rain will end tomorrow morning from west to east.
WEDNESDAY WARM-UP: The GFS is coming in even warmer for Wednesday… suggesting a partly sunny sky and a high of 71 degrees. The NAM is cooler with 64, and as usual, the truth is probably somewhere between those two values. But, one way or another spring-like warmth returns.
WEDNESDAY NIGHT-THURSDAY: We will forecast a good chance of showers and thunderstorms, with the main window for storms now coming from about 9:00 p.m. Wednesday through 12:00 noon Thursday. Shear values are getting more robust on the morning model runs, but the instability remains in question. SPC maintains the low end 5 percent severe weather probabilities for Alabama, and we will watch developments closely as this system approaches. Hopefully the timing will prevent a major severe weather issue with the storms coming through at a time when instability values tend to be lowest, during the late night and morning hours. Rain amounts of 1/2 to 1 inch look likely with this system.
FRIDAY AND THE WEEKEND: Friday will be dry and pleasant with a high in the low 60s. A low is expected to form over the northern Gulf of Mexico Saturday, and model output is not very consistent with this feature. The 12Z run of the GFS and the ECMWF keep the bulk of the rain over the southern half of Alabama, especially near the Gulf Coast. I still think we need to mention a chance of rain Saturday basically from I-20 south, and we can adjust the forecast as needed as the weekend gets closer.
Sunday will be dry and cooler with a high close to 50 degrees. Parts of North Alabama might not make it out of the 40s with a cool breeze out of the north.
LONG RANGE: Still no sign of any long lasting cold snap or winter mischief through the rest of February, but the GFS ensembles are starting to trend negative on the good ole NAO (North Atlantic Oscillation) as we approach the month of March. See the Weather Xtreme video for details and graphics.
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I had a great time today visiting with the 6th graders at North Jefferson Middle School… be looking for them on the Pepsi KIDCAM this afternoon at 5:00 on ABC 33/40 News. The next Weather Xtreme video will be posted by 7:00 a.m. tomorrow…
Winter Weather Advisory – TN Valley
The NWS Huntsville has issued a winter weather advisory for their county warning area up in the Tennessee Valley of extreme North Alabama. No doubt there will be some sleet and a few snowflakes up there for the next few hours, but temperatures are well above freezing, and quite frankly I don’t expect any travel issues or significant accumulation.
See the radar below…
We have had reports of sleet across a number of counties in West Alabama, and that will continue for a while. But temperatures at the surface are well above freezing, and the precipitation will change to all rain tonight without creating any driving problems.
Will have a full discussion and new Weather Xtreme video shortly.
One of the Coldest Airmasses in U.S. (and Alabama) History
On February 13, 1899, one of the coldest airmasses ever observed in the U.S. made it all the way to the Gulf Coast. It was 7 in New Orleans and Pensacola. Mobile dropped to a numbing -1. The reading of -2 at Tallahassee still is the state’s coldest reading ever. Many all time state record lows were observed during the cold wave.
In Birmingham, observations were taken at the old Fountain Heights weather office. According to J.B,, records were kept in a beat up old journal. The official low on this frigid morning was -10 at the weather office. Handwritten notes on the journal for the date indicated that the temperature in outlying areas around the city was -14.
If the reading had been taken at the current observation post at the airport, it would have surely been –14. If readings had been kept in Pinson then, (normally coldest in Birmingham area) it is safe to bet that the reading there would have been –17!
Other Alabama lows that cold morning: -7 in Tuscaloosa, Elba and Opelika; -5 in Greensboro; -11 in Florence; -12 in Decatur; -15 in Oneonta; -16 in Hamilton and Scottsboro and –18 in Valley Head. Greensboro had five inches of snow on the ground.
A major blizzard was spreading a wide swath of snow from Florida to Maine. Snow flurries were reported in Fort Myers, Florida. The blizzard, dubbed the “Storm King,” dumped nearly 16 inches of snow on New York City on top of an 11 inch snowcover. Twenty inches of snow fell at Washington DC and thirty four inches fell at Cape May, NJ. The pressure in the center of the storm was estimated at 966 Mb (28.53 inches), as strong as a major hurricane.
More cold records that fell during the coldwave included these all time records: -8 at Dallas, -16 at Amarillo, -23 Tulia for the coldest ever in Texas, -13 at Little Rock, -22 at Kansas City and -15 at Washington, DC.
